Transcript The Theory of Continental Drift

Continental Drift & Plate Tectonics
Continental Drift Theory
• First proposed by Alfred Wegener in 1912:
– 250 million years ago, all of the continents
were combined into one super-continent
called “Pangaea”
– The continents gradually drifted apart to
where they are today
• Wegner didn’t make up this theory out of
the blue – like all scientists, he based it on
evidence
Evidence Summary
• Geographic fit of South America and Africa
• Fossils match across oceans
• Rock types and structures match across
oceans
• Ancient glacial features
Geographic Fit
• Continents look like
they could be part
of a giant jigsaw
puzzle
• Here’s how they
moved apart
Fossils Match
• Plant and animal fossils found on the coastlines
of different continents
Fossil Fuel in Antarctica
• Tropical plant remains (coal deposits) found in
Antarctica
– this is evidence that Antarctica was once much warmer
and much closer to the equator, since tropical plants
don’t grow in Antarctica today
Rock Structures Match Across Oceans
• Same rock patterns found in South America,
India, Africa, Antarctica and Australia
Rock Structures Match Across Oceans
Ancient Glacial Features
• Glaciation in
Africa, South
America, India,
and Australia
during the same
time
Ancient Glacial Features
Evidence but no Method
• While Wegener presented compelling
evidence, there was still no explanation for
HOW the continents drifted.
• The question remained: “If continents drift,
what is making them move?”
Sea Floor Discoveries
• WW II: Military Spending
• U.S. Navy mapped seafloor with sonar in
order to help ships and submarines
navigate.
• They expected to find that the ocean floor
was a vast, flat plain. What they found was
shocking.
Sea Floor Discoveries
• Instead of miles and miles of flat surface,
they found that the ocean floor had:
– oceanic ridges - submerged mountain ranges
– fracture zones - cracks perpendicular to ridge
– trenches - narrow, deep gashes
– seamounts - drowned undersea islands
Sea Floor Discoveries
• In addition, they discovered that the rocks of the
seafloor included only basalt, gabbro, and
serpentinite - no continental materials.
• This suggested that the sea floor was not simply
“covered up” continental crust, but was made of
different materials and at a different time
Sea Floor Discoveries
• Further study of these rocks led scientists
to even more surprising information:
• The sea floor’s youngest rocks were
located right at the ocean ridge – and as
you moved away from the ridge in either
direction, the rocks got progressively
older.
Sea Floor Discoveries
Sea Floor Discoveries
• What scientists discovered was that the sea floor was
being constantly “recycled.” The youngest rocks were
created from magma rising to the surface, hardening and
pushing aside the older rock.
• Scientists called this process “sea floor spreading.”
Sea Floor Discoveries
• So now we know:
– sea floor is being created at the midocean ridges
– sea floor is spreading
– the oldest ocean floor occurs at the
coastlines of continents…
• Why doesn’t the earth
get bigger? Where does
the ocean floor go? Why
doesn’t it get any older?
Sea Floor Discoveries
• The ocean floor is pushed against the
continental crust – and because it is
denser, it dives under the crust.
• This process is
called
subduction
The Rise of Plate Tectonics
• In the late 1950’s, the United States was
engaged in The Cold War with the Soviets
• To keep an eye on Soviet nuclear tests, the
U.S. military developed new, advanced
seismometers
• These seismometers were deployed in over 40
allied countries and were recording 24 hrs/day,
365 days/year
The Rise of Plate Tectonics
• Besides nuclear tests, the seismometers
recorded every moderate to large earthquake
on the planet.
• Scientists mapped the earthquake data and
found something they weren’t expecting:
– Armed with this high-precision earthquake data,
seismologists found that activity happens in narrow
bands.
The Rise of Plate Tectonics
The Rise of Plate Tectonics
• The discovery of these bands led scientists to
understand that the earth’s outer shell is
broken into thin, curved plates that move
laterally atop a weaker underlying layer.
• Think of it like a hard-boiled egg: you can put
cracks all over the shell of a hard-boiled egg,
but the egg is still “whole”
Types of Plate Boundaries
• The interaction of the plate edges with each other
can be classified as one of three main types of
boundaries:
• Convergent boundaries
• Divergent boundaries
• Transform boundaries
Types of Plate Boundaries
• Convergent: areas of plates that are moving
toward each other
– there are three sub-types of convergent boundaries:
• oceanic to continental
• continental to continental
• oceanic to oceanic
Types of Plate Boundaries
• Divergent: areas of plates that are moving away
from each other
Types of Plate Boundaries
• Transform: areas of plates that are sliding past
each other
Types of Plate Boundaries
• Here’s an animation of each type of plate boundary
Types of Plate Boundaries
Sea Floor Discoveries
Bill Nye discusses sea floor discoveries and plate tectonics
Review the Facts
Continental
Drift
Sea Floor
Plate
Tectonics
1 pt
1 pt
1 pt
2 pt
2 pt
2 pt
3 pt
3 pt
3 pt
Go To Exit Slip
Continental Drift 1 pt
• The scientist who first proposed the
theory of continental drift
Continental Drift 1 pt Answer
• Who was Alfred Wegner?
Continental Drift 2 pt
• fossils match across continents and
oceans, same rock patterns found on
five different continents, mountain
ranges match across the Atlantic
ocean
Continental Drift 2 pt Answer
• What is some of the evidence
Wegner used to support his theory
that the continents were once joined?
Continental Drift 3 pt
• Because he had no answer to the
question, “If the continents are
drifting, what mechanism is causing
them to move?”
Continental Drift 3 pt Answer
• Why was Wegner’s theory of
continental drift widely ignored?
Sea Floor 1 pt
• ocean ridges, fracture zones, trenches,
seamounts
Sea Floor 1 pt Answer
• What did Harry Hess and the US Navy
discover when they mapped the sea floor
using sonar?
Sea Floor 2 pt
• The process of ocean crust “diving” under
other crust into the mantle
Sea Floor 2 pt Answer
• What is subduction?
Sea Floor 3 pt
• The process by which the newest ocean
floor is created at mid-ocean ridges,
pushing older crust outward
Sea Floor 3 pt Answer
• What is sea floor spreading?
Plate Tectonics 1 pt
• What the earth’s outer shell is broken into
Plate Tectonics 1 pt Answer
• What are plates?
Plate Tectonics 2 pt
• Instrument deployed by US Military to spy
on soviet nuclear tests which also detected
medium-to-large earthquakes.
Plate Tectonics 2 pt Answer
• What is a seismometer?
Plate Tectonics 3 pt
• The three ways plate boundaries interact
Plate Tectonics 3 pt Answer
• What are convergent, divergent, &
transform?
Review – Exit Slip
• Choose what you believe to be the strongest piece of
evidence Wegner uncovered to support the idea of
continental drift and explain why you think it is the most
convincing.
• If new sea floor is constantly being created, why isn’t the
earth growing in size?
• How did earthquakes help scientists to understand the
structure of the earth’s crust?
•
THE FINE PRINT
•
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Continental Drift and the Changing Oceans
Important Events – 225 mya
• Pangaea was surrounded by single ocean called Panthalassa which
would become the modern Pacific Ocean.
• Thethys Sea, the precursor of the Mediterranean Sea, separated
Eurasia from Africa.
• The Sinus Borealis would become the Arctic Ocean.
Important Events – 200 mya
• Rift appears between North America and combined South America
and Africa.
• Was the beginning of the Mid-Atlantic Ridge and created the North
Atlantic Ocean
• Pangaea was separated into 2 large continents – Laurasia (North
American and Eurasia) and Godwana (South America, Africa, India,
Antarctica, and Australia).
• At the same time, a rift began to separate India from Godwana
creating the Indian Ocean
Important Events – 135 mya
• South Atlantic Ocean was born as a rift separated South America
from Africa.
• This rift joined the mid-ocean ridge in the North Atlantic to form a
single Mid-Atlantic Ridge.
• The growth of the Atlantic Ocean caused the Americas to drift
further from Africa and Eurasia.
• To make room for the new seafloor created in the Atlantic, the
Pacific Ocean shrank.
Important Events – 65 mya
• The Y-shaped rift that created the Indian Ocean continued to grow
and worked to separate Australia from Antarctica.
• The base of the rift formed the Red Sea.
• India moved North until it crashed into Asia creating the Himalayas.
Records in the Sediments
• Scientists can tell a lot about the Earth’s
history by examining the types of
sediments laid on the ocean floor.
Marine Sediments
Lithogenous Sediment
Biogeneous Sediment
• Derived from weathering of
continental rocks
• Large particles sink while finer
material is carried away in
ocean currents.
• Coarse lithogenous sediments
are deposited near continental
edge.
• Consists of the skeletons and
shells of microscopic marine
organisms – radiolarians,
diatoms, foraminiferans
• AKA microfossils
• Can tell scientists what
organisms lived in the ocean in
the past and can indicate
ancient ocean temperatures
• Carbon dating can determine
age of microfossils
Biogenous Sediment
Sea Level Changes
Structure of the Sea Floor
• Because the ocean floor is
created/destroyed due to the workings of
plate tectonics, major features are similar
from place to place worldwide
Sea Floor
Continental Margins
Deep Sea Floor
• Submerged edges of
continents
• Boundaries between
continental crust and oceanic
crust
• Consist of continental shelf,
continental slope and
continental rise
• Most lies at a depth of 30005000 m
• Aka abyssal plains
• Dotted with abyssal hills
seamounts, guyots
• Trenches and ridges
• Hydrothermal vents
Continental Margin
Continental Shelf
• Shallowest part of the continental margin
• Biologically richest with most life and best
fishing
• Composed of continental crust that is
submerged
• Were exposed during past times of low
sea level
Continental Shelf and Politics
• The continental shelf is an underwater extension of
land that can stretch out to sea for many kilometres.
Government scientists are studying the Canadian
continental shelf in the Atlantic Ocean as part of the Extended
Continental Shelf (ECS) Program, a large initiative set up
to identify characteristics of the shelf under the Atlantic and
Arctic Oceans. The aim of the Program is to define the outer
limits of the shelf where it extends beyond 200 nautical
miles (NM) from coastal baselines, thereby determining with
precision where Canada may exercise its existing sovereign
rights over the natural resources of the seabed and subsoil.
The scientific data collected as part of this initiative will
be used for Canada’s submission to the United Nations
Commission on the Limits of the Continental Shelf (CLCS).
Continental Slope
• Begins at the shelf break and descends
down to the sea floor
• Water becomes colder and more sluggish.
• Oxygen levels decline; pressure increases
• Ocean life along the continental slope
must be much more tolerant of extreme
conditions.
Continental Rise
• Formed by sediments accumulating on the
sea floor at the base of the continental
slope
• The continental rise is often used as an
indicator by ships, since it indicates that
the continental shelf, and land, are close
by.
Nature of Continental Margins
Active Margin
Passive Margin
• Exists at plate boundaries
notably subduction zones
• Characterized by steep, rocky
shorelines and intese
geological activity
(earthquakes, volcanoes)
• Narrow continental shelf and
steep continental slope lead to
a trench or a poorly developed
continental rise
• Exists in geologically inactive
zone
• Characterized by flat coastal
plains
• Wide continental shelf and
gradual continental slopes, and
thick continental rises
Continental Margins
Abyssal Plains
• Underwater plain on the deep ocean floor, usually found at depths
between 3000 and 6000 metres.
• Lying generally between the foot of a continental rise and a midocean ridge, abyssal plains cover more than 50% of the Earth’s
surface.
• They are among the flattest, smoothest and least explored regions
on Earth.
• Seamounts and hydrothermal vents are found here
• Home to a tremendous variety of marine life
Biodiversity in the Deep Ocean
Seamounts
• Seamounts are
volcanic mountains
arising from the sea
floor.
• Some are flat-topped
and are called guyots
Bowie Seamount
•Bowie Seamount is the shallowest
submarine volcano in Canada
•Rises 3000 m from the ocean floor
to within 24 m of the water’s
surface
•Because of its biological richness,
Bowie Seamount was designated
as Canada's seventh Marine
Protected Area on April 19, 2008
under the Oceans Act
•To the Haida Nation, the
submarine volcano is called Sgaan
Kinghlas. In their language it means
"Supernatural Being Looking
Outward“
•Recognized as a hazard to
navigation and is avoided by
shipping vessels.
Hydrothermal Vents
•Commonly found near
volcanically active places,
areas where tectonic plates
are moving apart, ocean
basins, and hotspots
•May form features called
black smokers
•Biologically productive
•Chemosynthetic bacteria
form the base of the food
chain